The invention relates to an electrodeless low-pressure discharge lamp comprising a discharge vessel sealed in a vacuum-tight manner and having a discharge space containing an ionizable vapor and a rare gas, the discharge vessel having a protuberance protruding into the discharge space, and a body of soft magnetic material, which is surrounded by an electrical coil, the magnetic body and coil being provided in the probuberance.
Such an electrodeless low-pressure mercury discharge lamp is known from GB No. 2,133,612A.
Such electrodeless lamps are favorable because their discharge vessel has small dimensions as compared with commercially available low-pressure discharge lamps provided with electrodes. The light generated by the lamps can thus be more readily concentrated by means of a luminaire. Furthermore, disadvantageous effects of electrodes on the life do not occur in the lamps.
A disadvantage is that the body of soft magnetic material is surrounded for the major part by the discharge, as a result of which the temperature of said magnetic body becomes comparatively high. Soft magnetic materials, such as ferrites, are in fact sensitive to heat. Their specific magnetic losses increase with increasing temperature, while at elevated temperature the magnetic permeability starts to decrease. Due to these factors the efficiency of the lamp is low.